Filling compounds are used in high voltage power cables to prevent moisture from travelling along the interior of the cable core; for example, between the conductor and the insulation or between the insulation and metal shielding. High voltage power cables have layers of semiconducting plastic material on both the inner and outer surfaces of the insulation. The layer between the conductor and the insulation is commonly referred to as the "conductor shield"; and the layer between the insulation and the metal shield is referred to as the "insulation shield."
Filling compounds are most commonly used to seal the cable between the conductor and the conductor shield. With stranded conductors, the sealing compound can be applied between the strands of the conductor. The semiconducting filling compound of this invention can be applied over the conductor in such a way as to provide not only a sealant but also to serve as the conductor shield. It can also be applied between the insulation and the metal shield which surrounds the insulation.
It is extremely important to have a filling compound which has good flow characteristics which permit the compound to be extruded over the surface which it surrounds without forming any voids or other variations in the extruded layer. Voids or other irregularities cause localized concentration of electrical stress, and such stress concentrations often result in the formation of electro-chemical trees, if there are traces of moisture present. Over a period of time, electrochemical trees cause deterioration and eventual breakdown of the insulation.
Amorphous polypropylene has unusually good flow characteristics when extruded on an underlying surface in the manufacture of high-voltage power cables. Additions of carbon black sufficient to obtain a satisfactory semiconducting blend do not impair the flow characteristics.
The only disadvantage that has been found in the use of amorphous polypropylene has been that it becomes brittle at temperatures below 0.degree. C. The compound can be made to withstand lower temperatures without becoming brittle if the amorphous polypropylene is blended with limited amounts of a low molecular weight polyisobutylene rubber or with a low molecular weight copolymer of isobutylene-isoprene rubber.
Where the brittleness of the compound at low temperatures is not a factor to consider, the amorphous polypropylene is used without blending in any other material except as much carbon as is necessary for the desired conductivity where a semiconducting filler is desirable. Carbon black or powdered graphite can be used to make the compound semiconducting. The amount of carbon required for semiconducting shields of high-voltage power cables does not impair the flow characteristics of the amorphous polypropylene to any significant extent.
When it is necessary to have the filler compound of this invention withstand lower temperatures, without becoming brittle, the amorphous polypropylene is blended with as much low molecular weight polyisobutylene rubber as is necessary to permit the filler to withstand lower temperatures. A blend of amorphous polypropylene and low molecular weight polyisobutylene rubber in equal parts by weight enables the filler compound to withstand a temperature as low as -11.degree. C.
The preferred blend, for low temperature work, is 371/2% amorphous polypropylene and 621/2% low molecular weight polyisobutylene rubber. This gives a brittle point of -16.degree. C. With an additional 10 parts of hydrocarbon oil, the blend has a brittle point of -22.8.degree. C.
Further reduction of the amount of amorphous polypropylene in the compound impairs the flow characteristics and, if the proportions of amorphous polypropylene to polyisobutylene rubber is reduced below a ratio of 25 to 75, it becomes necessary to add a processing aid to the blend in order to maintain satisfactory flow characteristics for the compound. Various plasticizers can be used as a processing aid, such as hydrocarbon oil, chlorinated paraffin, isobutylene, liquid plasticizer, or low molecular weight polyethylene. The amount of processing aid added to the blend should be limited to that necessary to restore the good flow characteristics of the blended material.
The amorphous polypropylene is preferably used without blending in other polymers, as previously explained, but whether used alone or in a blend, the amount of carbon black or graphite material used to make the polymer semiconducting may be within the range of from 40 to 150 parts of carbon to 100 parts of the polymer, whether amorphous polypropylene is used alone or blended with other materials, as already described. All mixture ratios as described herein are by weight.
The filler compound of this invention retains its adhesiveness, consistency and resistivity, as demonstrated by long term oven aging at 121.degree. C. It retains its moisture resistance after long term water immersion at 80.degree. C; is nondripping at 121.degree. C, and it is a non-migrating compound or at least a non-contaminating composition.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.